Test and measurement is a critical part of product and system development both at the prototype stage as well as during production. Increasingly, many products and systems employ high frequency microwave and millimeter wave electrical signals. These include modern optical communication systems, radar, electronic warfare, sensors, etc. Testing the performance and functionality of these systems, both at the component level and the system level, requires advanced high frequency test equipment.
The most common methods for RF device and system test and measurement include the use of vector network analyzers (VNAs) or RF spectrum analyzers.
Electro-optic modulator test and measurement has primarily been accomplished using either an optical spectrum analysis approach or various RF in to RF out based approaches. One of the most common RF in to RF out approaches involves using a vector network analyzer and a calibrated photodetector to measure the response of an electro-optic modulator.
The primary deficiencies with optical spectrum analysis based approaches for electro-optic modulator characterization are that they are slow, expensive, often low resolution, and have limited wavelength ranges. They are slow because the entire optical spectrum must be scanned at each RF frequency point. In grating based optical spectrum analyzers spectrum acquisition alone can be a lengthy process. Following acquisition, the trace that is collected must be processed to determine the amplitudes of the various optical sidebands to compute the electro-optic modulator response at that frequency. Optical spectrum analyzers are expensive pieces of test equipment with state of the art instruments in the $100k range. Grating based optical spectrum analyzers (OSAs) typically offer a wider wavelength range than other types of OSAs, but their frequency resolution is limited by their gratings. This limits the utility of these devices for measuring low RF frequencies. High resolution OSAs typically only operate over a limited wavelength range.
The primary deficiency with RF based approaches for electro-optic modulator characterization is that the most common approaches require both a calibrated RF VNA and a calibrated high speed photodetector. While RF VNAs are excellent instruments, calibrated photodetectors are less reliable. In addition, both of these units are expensive, and calibrated high speed photodetectors may not be available at certain wavelengths. The prior art includes other more novel techniques to avoid using high speed detectors. Some of these approaches utilize one or more tunable laser sources to optically mix the sidebands generated by an electro-optic modulator down to low RF frequencies where they can be measured using a low speed detector. It is clear that using multiple lasers and tunable lasers increases the cost and complexity and reduces the reliability of the measurement system.
The closest all optical approach is U.S. Pat. No. 6,204,954. In this patent, the inventor measures the reduction in peak intensity when a Mach Zehnder modulator biased at peak is driven with an RF signal and uses this to determine the Vπ of the modulator. This approach uses only low speed detectors, but it has a limited sensitivity due to the fact that the optical power that is detected is dominated by the optical carrier. Essentially they measure a small signal on a large background.
Needs exist for improved test and measurement of electro-optic modulators and RF devices and systems.